Here's a website I've been working on: http://rifflist.org/ its basically a site where you can post an idea for some movie, book, experiment, business, game etc... People can browse through the list for inspiration, share their own ideas to get them out of their head, and comment on ideas to riff on them. Still haven't figured out how to make a backend for it though :P, so adding a comment is all client-side right now
Ooh, this reminds me of Workflowy, but you can't collaborate on that site at all.
I've used Workflowy since it was in early beta, and have not found any tool more useful. Great site. Great people. Great product. They also got it to GREAT and then left it. They have the perfect minimalist tool, and they have not overburdened it with useless crap and features and interface re-works. It has been the same for ... I dunno... 7 years now? Love it.